The present invention relates to an apparatus and method for spray painting of an article, such as an automobile body, with a paint having a high viscosity, where the paint is heated to reduce the viscosity and enable ready spray application by an electrostatic spray device.
The spray painting of an article, such as an automobile body, is often carried out by use of electrostatic spray devices and uses a variety of paint compositions. Generally, such paint compositions have high viscosities and the viscosity must be lowered for use in electrostatic spraying by either dilution with a solvent or diluent or heated to a temperature at which the viscosity is low enough for application. Both such procedures provide further problems. The use of solvents or diluents provides unwanted volatile components that must be contained and disposed of, or recovered, adding to the cost of the paint system, and also can cause problems in the application of the paint to a surface, such as runs, drips, sags, solvent popping or trapping, or the like. The heating of the paint composition, while reducing the amount of unwanted additional components such as solvents or diluents, and reducing the viscosity of the composition, can lead to problems where the paint composition is unstable if exposed to heat for any extended period of time, and if spraying is interrupted, the heated paint composition may begin to react or form a gel.
An example of a prior art paint system for an automotive plant where heating is used to reduce the viscosity of the paint composition is given in U.S. Pat. No. 5,395,541 which shows the use of a plate type heat exchanger for each circulation loop, with the heat exchanger connected in parallel in a single water circulation loop. Another prior art paint temperature control system using a heat exchanger is described in U.S. Pat. No. 3,880,228. In the system described in U.S. Pat. No. 3,880,228, a heat exchanger zone is used with means for continuously circulating a heat exchange fluid into and through the heat exchanger zone and means are also provided for directing a relatively small stream of paint into heat exchange relationship with the fluid, such as hot oil, in the heat exchange zone, the mass velocity of the paint stream being substantially smaller than the mass velocity of the hot fluid. The paint is thus heated only once as it passes to a painting apparatus from the heat exchanger, but the paint is constantly circulated at ambient temperature to and from a paint reservoir.